1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for plasma processing of a substrate and, more specifically, to a method and apparatus for etching a substrate using a stable plasma.
2. Description of the Related Art
Integrated circuits have evolved into complex devices that can include millions of transistors, capacitors and resistors on a single chip. The evolution of chip designs continually requires faster circuitry and greater circuit density. Circuit density has a pronounce importance as the speed and number of functions a circuit can execute increases along with the density of the circuit structure. Some design attributes affecting the speed and circuit density of integrated circuits include the resistance and thickness of the materials used to form the layer comprising the circuit structure formed on a substrate.
Metallic materials are used to create wireline interconnects, vias, electrodes, and the like. The metal structures are key to the functionality of an integrated circuit. One metal that is frequently used to fabricate circuit structures is tungsten. Tungsten may be accurately deposited using conventional Chemical Vapor Deposition (CVD) methods and generally has a low resistivity. Circuit designers have found tungsten to be a favorable material for use proximate polysilicon as tungsten exhibits good resistance to permeation by polysilicon, which enables tungsten to retain its physical properties over the course of substrate processing and device use.
In order to maximize circuit density, the layers comprising the circuit structure, including those comprising tungsten, must be minimized. However, when processing such thin layers, care must be taken to avoid damaging the layers during processing. Damaged layers result in defective circuit structures and increased substrate rejects.
One process that can easily damage thin layers is etching. Fluorinated chemistry is typically employed to remove exposed tungsten and other metals. A plasma is utilized to enhance the etch process. However, it is difficult to maintain a stable plasma. One method for increasing plasma stability is to increase the power supplied to the chamber. Another method is to decrease the gap between the substrate being etched and the top of the chamber. Unfortunately, both increasing the power and decreasing the gap lead to an increase in ion bombardment of the substrate, which may seriously damage the circuits being formed on the substrate.
Therefore, there is a need in the art for an improved method and apparatus for etching metals, especially tungsten.